1. Field of the Invention
This invention relates to an infrared detector for scanning the temperature of the passing wheels of railroad cars and, more particularly, to a self-calibration system therefor.
2. Description of the Prior Art
It is well-known that overheated railroad wheels, typically caused by a defective brake mechanism or by improperly prolonged brake application, can lead to serious malfunctions of railroad equipment--including thermal cracks in the wheel which may extend into the wheel plate, and softening of wheel rims which leads to non-circular wheels (i.e., "flats") which can damage wheel bearings and crack rail sections--and result in fuel inefficiencies, costly field repairs, and even derailment.
It is well-known in the prior art to scan passing railroad wheels with an infrared-type apparatus to detect overheated railroad wheels before the aforementioned serious consequences occur. However, calibration of this apparatus due to changing sensor characteristics is required to maintain accurate readings. In the prior art, this calibration is done manually by simply varying the gain of an analog amplifier under test conditions until the desired result is achieved. This is deficient in that a high skill level is required of the technician to achieve an accurate result. This is further deficient in that the external adjustment to the amplifier gain, typically through a potentiometer, makes the apparatus vulnerable to vandalism.
Moreover, the prior art uses analog measurement devices associated with the sensor. Such devices use analog comparators for alarming devices which do not satisfactorily account for the differences in compensation required for scanning wheels on both the near and far rails due to differences in distance between the rails and the scanner. Similarly, such devices typically do not indicate the precise location of the overheated wheel.